router back to back connections

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Jun 5th, 2008
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Hi all, we have 2 routers using glbp, and bgp for the wan connection, my question is, they have 1 connection each to the lan, and one connection plugs directly into each other, what is this used for ?

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Rajat Chauhan Thu, 06/05/2008 - 06:51
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That actually depends on the design, and boils down to the configuration.

Majorly, its used as a backup for FHRPs ( HSRP, VRRP, GLBP etc) in case we lose any of the uplinks.

Secondly, it prevents traffic blackhole in certain scenarios due to node/link failure.

AGain, it depends how its configured. The scenarios descried above works only if its in L2-mode.

Alternatively, it can be used as L3 link for route maintenance and proper route summarization towards core.

Most modern networks utilize a hybrid form of it, combining L2/L3 modes and getting best of both worlds.



carl_townshend Thu, 06/05/2008 - 07:45
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Im still unsure what you mean here, can you explain, I thought that if using a redundancy protocol both connections have to use the lan port, would the middle cable be more for routing ?

Rajat Chauhan Thu, 06/05/2008 - 09:34
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Right, for FHRP hellos, it uses lan link by default, but for predictable designs, lets say only a link loss is there , but the active HSRP router is still active and I dont want HSRP to failover and make 2nd one active (there can be numerous scenarios), I'll use dist-link to pass hellos.

Please refer to some design docs or CCDP curriculum for further details.

From a L3 point of view, its critical for route summarization and avoiding of traffic blackhole due to link failure.



carl_townshend Sat, 07/26/2008 - 00:50
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Hi there, so, how do you make the hsrp hello's go through the dist link? i thought they would only talk on the lan link? also what would the routing protocol do with this link?



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