Difference between revisions of "Manual:BGP HowTo & FAQ"
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Revision as of 11:58, 16 September 2009
- 1 Problem: BGP is not connecting
- 2 Problem: BGP connection is established, but routing updates are ignored
- 3 Question: How to check if a specific route exists in IP routing table?
- 4 Problem: Routes are exchanged and installed in IP route table, but they stay inactive
- 5 Question: How to filter out something?
- 6 Question: How to quickly check how many routes there are in route table?
- 7 Question: How to seen routes advertised to, and routes received from a particular peer?
- 8 Question: Is load balancing possible with MT BGP?
- 9 Question: How to announce routes?
- 10 Question: What does BGP network synchronize option exactly mean?
- 11 Question: How to control advertised routing information?
- 12 Problem: Looks like my routing filter isn't working
- 13 Question: How to announce just a single large IP prefix instead of many smaller (i.e. more specific) prefixes?
- 14 Question: How to aggregate IGP routes?
- 15 Question: How to advertise the default route?
- 16 Problem: Routes are announced, but with attributes not from IP routing table
- 17 Question: Can MT propagate BGP route updates without installing them in IP route table (i.e. serve as a pure route reflector)?
Problem: BGP is not connecting
- BGP uses TCP, you can start with testing TCP connectivity. One way to do that is as simple as /system telnet <remote-ip> 179. The result you see should be something like this: "Connection closed by foreign host". This means BGP port (179) is open and reachable.
- If this is eBGP, make sure you have configured multihop=yes and TTL as needed. Use /routing bgp peer print status to see the current state of BGP connection.
- Also note that if the remote peer is not supporting BGP Capabilities Advertisement (RFC 2842), some extra time will be needed for session establishment.
Problem: BGP connection is established, but routing updates are ignored
- NLRI (Network Layer Reachability Information) is ignored if path attributes are invalid. Turn on BGP debug logs to see the exact cause of the problem. (/system logging add topics=bgp,!raw).
- One frequent case is unacceptable BGP next-hop. (Read here more about RouterOS and BGP next-hops.) In this case you must fix the next-hop on the sending side. In case the sender also is MT, you can use nexthop-choice peer setting to modify default next-hop selection preferences. If that fails, specify next-hop manually using set-out-nexthop routing filter.
Question: How to check if a specific route exists in IP routing table?
- Finding a route by prefix is pretty fast:
/ip route print where dst-address = 18.104.22.168/24
- To find all routes with prefixes falling in a range:
/ip route print where dst-address in 22.214.171.124/16
- You can also search routes by other attributes, but it will be much slower and can take some time on a router having full BGP feed.
- For example, since RouterOS 3.23 you can use this syntax to match routes having originated from a specific AS 30621:
[atis@SM_BGP] > /ip route print detail where bgp-as-path ~ "30621\$" Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, B - blackhole, U - unreachable, P - prohibit 0 ADb dst-address=126.96.36.199/23 gateway=x.x.x.x recursive via y.y.y.y ether1 distance=20 scope=40 target-scope=10 bgp-as-path="2588,42979,702,701,7018,30621" bgp-origin=igp received-from=x.x.x.x 1 ADb dst-address=188.8.131.52/22 gateway=x.x.x.x recursive via y.y.y.y ether1 distance=20 scope=40 target-scope=10 bgp-as-path="2588,42979,702,701,7018,30621" bgp-atomic-aggregate=yes bgp-origin=igp received-from=x.x.x.x
Problem: Routes are exchanged and installed in IP route table, but they stay inactive
- Routes must be resolved to become active; it's possible that you need to change scope or target-scope attributes for some routes.
Question: How to filter out something?
- Use routing filters. For example, to filter out routes with a specific BGP community, add this rule:
/routing filter add bgp-communities=111:222 chain=bgp-in action=discard
- Then tell BGP peer to use that filter chain:
/routing bgp peer set peer in-filter=bgp-in
- There is also an out-filter BGP peer parameter for filtering outgoing BGP updates.
- Note that since RouterOS 3.16, using routing-test package, bgp-as-path filter accepts regular expressions.
Question: How to quickly check how many routes there are in route table?
- For all routes use:
ip route print count-only
- To see route count from a particular peer look at prefix-count property in:
route bgp peer print status
Question: How to seen routes advertised to, and routes received from a particular peer?
- To see routes advertised to a particular peer (equivalent to show ip bgp neighbor x.x.x.x advertised-routes) use:
routing bgp advertisements print
routing bgp advertisements print <peer_name>
- To see routes received from a particular peer (equivalent to show ip bgp neighbor x.x.x.x received-routes) use:
ip route print where received-from=<peer_name>
Question: Is load balancing possible with MT BGP?
- Yes. Even though BGP itself cannot propagate multiple next-hops for a single route through the network, there are ways how to have routes with multiple next-hops on a router.
- One way is to set multiple next-hops with routing filter.
routing filter add chain=bgp-in set-in-nexthop=10.0.1.1,10.0.2.1
- Another way is to resolve BGP next-hop (if it is not directly reachable) through a static or OSPF route with multiple next-hops.
ip route add dst-address=x.x.x.x/y gateway=10.0.1.1,10.0.2.1
Question: How to announce routes?
- If your don't have many routes to announce and want the best control over them, use BGP networks or aggregates. Note that both maximal BGP network and aggregate count is limited to 200.
- Otherwise use route redistribution options, configurable under BGP instance settings.
Question: What does BGP network synchronize option exactly mean?
- Since version 3.30 routing-test it means "do not announce this network, unless there is a matching active IGP or connected route in IP route table".
Question: How to control advertised routing information?
- Use routing filters.
- To advertise the same information (e.g. some BGP attribute value) to all peers, use BGP instance out-filter:
/routing filter add set-bgp-communities=111:222 chain=bgp-out /routing bgp instance set default out-filter=bgp-out
- To send routing information to different peers, use peer specific filters. For example, if you want to advertise a lower preference value (higher path cost) to one of the peers, you can prepend your AS number multiple times to the BGP AS_PATH attribute:
/routing filter add set-bgp-prepend=4 chain=bgp-out-peer1 /routing bgp peer set peer1 out-filter=bgp-out-peer1
- Use /routing bgp advertisements print to see what routing information exactly is advertised to peers.
Problem: Looks like my routing filter isn't working
- Most likely prefix matcher is configured incorrectly. For example, say that you want to configure filter that will discard all routes falling under prefix 184.108.40.206/24.
- The correct way to do this is with specifying prefix-length matcher:
add prefix=220.127.116.11/24 prefix-length=24-32 action=discard chain=bgp-in
- Or (the same effect):
add prefix=18.104.22.168 prefix-length=24-32 action=discard chain=bgp-in
- This rule is incorrect (default netmask is /32, so it will match only prefix 22.214.171.124/32):
add prefix=126.96.36.199 action=discard chain=bgp-in
- This is incorrect too (because it will match only route with netmask 255.255.255.0)
add prefix=188.8.131.52/24 action=discard chain=bgp-in
- Use filter action log to see which routes are matched by a routing filter.
Question: How to announce just a single large IP prefix instead of many smaller (i.e. more specific) prefixes?
- Use BGP aggregates if you need to aggregate multiple routes in a single one. An aggregate will be announced one if there are some active routes with more specific netmasks falling under it. When an aggregate becomes active, a corresponding blackhole route is a automatically created.
- By default, BGP aggregates take in account only BGP routes. To also include IGP and connected routes in consideration, use include-igp configuration option.
Question: How to aggregate IGP routes?
- Since 3.30 you can specify include-igp in BGP aggregate configuration. Example:
ip route add dst-address=10.9.9.0/25 gateway=10.0.0.1 ip route add dst-address=10.9.9.128/25 gateway=10.0.0.2 routing bgp aggregate add instance=default prefix=10.9.9.0/24 include-igp=yes
[admin@MikroTik] > routing bgp advertisements print PEER PREFIX NEXTHOP AS-PATH ORIGIN LOCAL-PREF peer1 10.9.9.0/24 10.0.0.131 incomplete
- Use routing filters to control which routes are aggregated. For example, if you don't want to aggregate connected routes:
routing filter add chain=aggregate-out protocol=connect action=discard routing bgp aggregate set [find] advertise-filter=aggregate-out
Question: How to advertise the default route?
- To send default route to a particular peer, set default-originate=always or if-installed for that peer.
Problem: Routes are announced, but with attributes not from IP routing table
- There exists a limitation in MT BGP operation: if a BGP network with synchronization turned off, or default route generated by default-originate=always configuration statement is announced, the attributes of that route will not be taken from routing table.
- If synchronize=yes or default-originate=if-installed is used, the attributes of the announced route will be taken from routing table.
Question: Can MT propagate BGP route updates without installing them in IP route table (i.e. serve as a pure route reflector)?