Hi friends
Here something strange I have noted. Please
have a look on this scenario.
R4#sh ip bgp regexp ^$
BGP table version is 19, local router ID is
150.4.4.4
Status codes: s suppressed, d damped, h
history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? -
incomplete
Network Next Hop Metric LocPrf Weight Path
*> 4.1.0.0/24 0.0.0.0 0 32768 i
*> 4.2.0.0/24 0.0.0.0 0 32768 i
*> 4.3.0.0/24 0.0.0.0 0 32768 i
*> 44.4.0.0/22 0.0.0.0 32768 i
s> 44.4.1.0/24 0.0.0.0 0 32768 ?
s> 44.4.2.0/24 0.0.0.0 0 32768 ?
s> 44.4.3.0/24 0.0.0.0 0 32768 ?
Lets have a look only on 4.x.0.0/24
network. R4 advertising this prefix to R5 and R3 as they are eBGP peers
Again R3 will advertise this same prefix to
R5.
Now we knew R5 has two paths to reach
4.x.0.0/24 prefix. One via R4 directly and another via R3.
Lets have a look on R5 bgp routing table.
R5#sh ip bgp
BGP table version is 13, local router ID is
150.5.5.5
Status codes: s suppressed, d damped, h
history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? -
incomplete
Network Next Hop Metric LocPrf Weight Path
*
4.1.0.0/24 155.1.35.3 0 100 400 i
*> 150.4.4.4 0 0 400 i
*
4.2.0.0/24 155.1.35.3 0 100 400 i
*> 150.4.4.4 0 0 400 i
*
4.3.0.0/24 155.1.35.3 0 100 400 i
*> 150.4.4.4 0 0 400 i
Its fine R5 selecting the best and valid
path as shortest AS path.
But now lets change the R5 and R4 eBGP peer and make it eBGP peer with ttl security
option.
R5#sh ip bgp
BGP table version is 17, local router ID is
150.5.5.5
Status codes: s suppressed, d damped, h
history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 4.1.0.0/24 155.1.35.3 0 100 400 i
* 150.4.4.4 0 0 400 i
*> 4.2.0.0/24 155.1.35.3 0 100 400 i
* 150.4.4.4 0 0 400 i
*> 4.3.0.0/24 155.1.35.3 0 100 400 i
* 150.4.4.4 0 0 400 I
Interesting isn't it???
R5 using the best path via the longest
path. Please tell me why this behavior where I am lacking to understand this
concept.
Lets check more deep as far my knowledge
R5#sh ip bgp 4.1.0.0 255.255.255.0
BGP routing table entry for 4.1.0.0/24,
version 14
Paths: (2 available, best #1, table
Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
1
100 400
155.1.35.3 from 155.1.35.3 (150.3.3.3)
Origin IGP, localpref 100, valid, external, best
400
150.4.4.4
(inaccessible) from 150.4.4.4 (150.4.4.4)
Origin IGP, metric 0, localpref 100, valid, external
We can say it is inaccessible so its not
using the path via R4 but wait lets check the reachability.
R5#ping 150.4.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to
150.4.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5),
round-trip min/avg/max = 40/82/124 ms
R5#traceroute 150.4.4.4
Type escape sequence to abort.
Tracing the route to 150.4.4.4
1
155.1.45.4 36 msec * 4 msec
R5#sh ip route 150.4.4.4
Routing entry for 150.4.4.0/24
Known via "eigrp 1", distance 90, metric 2297856, type
internal
Redistributing via eigrp 1
Last update from 155.1.45.4 on Serial1/1, 00:33:12 ago
Routing Descriptor Blocks:
*
155.1.45.4, from 155.1.45.4, 00:33:12 ago, via Serial1/1
Route metric is 2297856, traffic share count is 1
Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
Why this behavior in ttl security peer but
when it was in ebg peer using multi-hop command it was choosing the shortest AS
path.
Please help me friends. Am breaking my head to understand this concept.
Thanks
Annamalai
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