OSPF: MTU XR vs IOS

Kejadian unik ketika troubleshoot link, saat itu OSPF antara ASR9K dan ME3750 tidak mau establish dengan benar. Si IOS XR menampilkan proses OSPF (Exstart/Exchange). Padahal saat show interface gi1/1/1 di sisi C3750, si mtu bernilai 9000.

C3750#sh int gi1/1/2
GigabitEthernet1/1/2 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is xxx.yyyy
Description: Link to ASR9010
Internet address is xx.xx.xx.xy/30
MTU 9000 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive not set
Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseZX SFP
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of “show interface” counters 40w6d
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
30 second input rate 5000 bits/sec, 6 packets/sec
30 second output rate 28000 bits/sec, 39 packets/sec
23005375103 packets input, 7147388758249 bytes, 0 no buffer
Received 15814942 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1 input errors, 1 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 15814937 multicast, 0 pause input
0 input packets with dribble condition detected
7178930024 packets output, 5735711202294 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out

C3750#

di sisi ASr9010, juga dibuat mtu=9000. Ternyata, yang dipake dalam proses OSPF itu bukan yang 9000 tapi yang lain. Maka jika menggunakan “show ip interface gig1/1/2” terlihat bahwa mtu=1500.

C3750#sh ip  int gi1/1/2
GigabitEthernet1/1/2 is up, line protocol is up
Internet address is xx.xx.xx.xy
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.5 224.0.0.2
Outgoing access list is not set
Inbound  access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP Flow switching is disabled
IP CEF switching is enabled
IP CEF switching turbo vector
IP Null turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Probe proxy name replies are disabled
Policy routing is disabled
Network address translation is disabled
BGP Policy Mapping is disabled
Input features: MCI Check
Output features: Check hwidb
C3750#

Sehingga disetinglah mtu di ASR9010 menjadi 1500.

ASR9K#sh run int gi0/0/0/1

interface gig0/0/0/1

mtu 1500

end

Ternyata OSPF masih belum establish. Statusnya masih sama. Hmm, usut punya usut si ASR perlu ditambah 14 bytes sehingga menurut [1] mesti ditambahkan 14 byte:

ASR9K#sh run int gi0/0/0/1

interface gig0/0/0/1

mtu 1514

end

Baru deh si OSPF establish.

ASR9K#sh ospf neig

[1]https://supportforums.cisco.com/document/58071/differences-mtu-configuration-between-ios-and-ios-xr

 

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