Routing Questions

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Revision as of 12:09, 19 February 2008 by Eep (talk | contribs) (Protected "Routing Questions": will be in manual [edit=sysop:move=sysop])
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Question: How does /ip route check-gateway work?

check-gateway sends pings every 10 seconds. if two successive pings fail, the gateway is considered dead.

Question: I have one /24 network advertised to two BGP peers using "/routing bgp networks" facility. How do I advertise a higher path cost to one of the peers?

You have to change the way you are redistributing your network, as filters are not applied to routes advertised from "/routing bgp networks". In most cases the network is connected directly to your router, so it's enough to set BGP instance to redistribute directly connected routes:

/routing bgp instance set default redistribute-connected=yes
To filter out all other connected networks except the needed one, create a routing filter for the BGP instance,
/routing filter add invert-match=yes prefix= action=discard name=InstanceOutFilter
then set filter "InstanceOutFilter" as the out-filter for "default" BGP instance.
/routing bgp instance set default out-filter=InstanceOutFilter
To communicate a lower preference value (higher path cost) to one of the peers, you have to prepend your AS number multiple times to the BGP AS_PATH attribute
/routing filter add prefix= set-bgp-prepend=4 name=Peer1OutFilter
/routing bgp peer set Peer1 out-filter=Peer1OutFilter

Question: I have a /22 (say assigned IP space, split internally down into /30's, /28's, etc. Is it possible just to announce the /22 space via BGP with routing-test package?

Yes, it is possible. Do the following:

1. add an empty bridge interface:
/interface bridge add name=loopback
2. assign a /22 address to the bridge interface:
/ip address add address= interface=loopback
3. create a routing filter that filters out all prefixes except the /22 one
/routing filter add invert-match=yes prefix= prefix-length=22 action=discard name=myfilter
4. set filter "myfilter" as the out-filter for "default" BGP instance
/routing bgp instance set default out-filter=myfilter

Question: How to blackhole a network?

There are two ways to blackhole a network. First, you can do this manually by adding a blackhole route to the routing table, for example, to blackhole a network, issue the following command:

/ip route add dst-address= kernel-type=blackhole

Routing filters are the other mean to blackhole a network. To create a routing filter that automatically blackholes all prefixes in in the BGP feed, issue the following command:

/routing filter add prefix= prefix-length=8-32 set-kernel-type=blackhole chain=myfilter

Question: How to filter out the default route from outgoing BGP advertisements?

Assuming you have a static default route that is redistributed because redistribute-static parameter is set to yes, do the following:

/routing filter add chain=myfilter prefix= action=discard

Then set myfilter as the out-filter for BGP instance

/routing bgp instance set default out-filter=myfilter