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List:       cisco-nsp
Subject:    [c-nsp] ebgp - multihop - prefixes received are inaccessible (next hop)
From:       CiscoNSP List <CiscoNSP_list () hotmail ! com>
Date:       2017-02-09 11:40:53
Message-ID: MWHPR13MB185469A229C8852BED7A0F388A450 () MWHPR13MB1854 ! namprd13 ! prod ! outlook ! com
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Hi,

Have a customer with 2 multiop eBGP peering sessions to 2 x ASR1001 - Sessions \
establish (2 hops away), they receive default, and advertise a /24 - we receive the \
/24, but prefixes "were" inaccessible and not added to routing table.

Setup is not really ideal, but cust is peering to our ASRs loop, and we are peering \
to an IP that is in a /29 that we have assigned to them (So the /29 to there bgp \
router, has an IP on an Int in our network...i.e. there def gw))

I say were, as I "fixed" one session....I knew the prefixes were inaccessible due to \
next hop, but next hop route was in bgp, and fib...from reading, bgp checks the int \
next-hop was learned from, and if the prefix was learned via a different Int (From \
peering Int), then you can get innaccessible?

What I found strange about this was that particular scenarion would be fairly common \
in ebgp multihop? i.e multiple redundant paths, the next hop could come from any of \
them if an outage occurred?

Anyway, even though the ASR was learning the next hop address via iBGP, and the \
address was in fib, it still complained with:

 BGP(0): xxx.xxx.xx.236 rcvd UPDATE w/ attr: nexthop xxx.xxx.xx.236, origin i, merged \
path xxx, AS_PATH  BGP(0): xxx.xxx.xx.236 rcvd yyy.yyy.yyy.0/24
 BGP(0): no valid path for yyy.yyy.yyy.0/24

I thought ebg-multihop would accomodate for this, but obviously it doesnt...so, I \
tried setting a static route for the /29 on one of the ASR's, to the loop of the PE \
they connect to...this worked, and the prefix learned went from innacessible to being \
placed in fib.

So, thought it would be simply a matter of doing the same on the other asr, but \
unfortunately, not...

The only difference between the 2 ASR1000's is that one is an RR...this is the one \
that is not working, and Im not sure if it being an RR is the cause..

Tried adding static route on the ASR1000 (RR)...made no difference, prefix still \
innacessible....why it doesnt like the prefix that is already in bgp/fib. not \
entirely sure, but even more puzzling is why the static route work-around doesnt work \
on this AST1K....the setup is identical ASR1k->PE(ASR920)->CUST in both \
locations...only difference is that the one that refuses to work is an RR

The one that doesnt work, I get this in debug:

BGP(0): xxx.xxx.xx.236 rcvd UPDATE w/ attr: nexthop xxx.xxx.xx.236, origin i, merged \
path xxx, AS_PATH BGP(0): xxx.xxx.xx.236 rcvd yyy.yyy.yyy.0/24
BGP(0): no valid path for yyy.yyy.yyy.0/24

The one with static, that does work, I see this:

BGP(0): (base) xxx.xxx.xx.124 send UPDATE (format) 0.0.0.0/0, next zzz.zzz.zz.201, \
metric 0, path Local BGP(0): xxx.xxx.xx.124 rcvd UPDATE w/ attr: nexthop \
xxx.xxx.xx.124, origin i, merged path xxx, AS_PATH BGP(0): xxx.xxx.xx.124 rcvd \
                yyy.yyy.yyy.0/24
BGP: nbr_topo global xxx.xxx.xx.124 IPv4 Unicast:base (0x7F9B7F521BE0:1) NSF rcvd \
                End-of-fib
BGP: nbr_topo global xxx.xxx.xx.124 IPv4 Unicast:base (0x7F9B7F521BE0:1) NSF \
Receiving router rcvd End-of-fib BGP(0): Revise route installing 1 of 1 routes for \
yyy.yyy.yyy.0/24 -> xxx.xxx.xx.124(global) to main IP table


Config (Basically identical on both ASR1K's)

 neighbor xx peer-group
 neighbor xx remote-as xxx
 neighbor xx CUST_MULTIHOP_TEST
 neighbor xx update-source Loopback0
 neighbor xxx.xxx.xx.236 peer-group xx
 neighbor xxx.xxx.xx.236 ebgp-multihop 2
 address-family ipv4
 neighbor xx next-hop-self
 neighbor xx default-originate
 neighbor xx soft-reconfiguration inbound
 neighbor xx route-map CUST-BGP-IN in
 neighbor xx route-map CUST-BGP-ADV out
 neighbor xx maximum-prefix 10 50
 neighbor xx filter-list 50 in


Any help is greatly appreciated :)


Thanks.

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